Future wireless communication networks will provide broadband services such as wireless Internet access to subscribers. Those broadband services require reliable and high-rate communications over hostile mobile environments with limited spectrum and intersymbol interference (ISI) caused by multipath fading. Orthogonal frequency division multiplex (OFDM) is one of the most promising solutions to address the ISI problem. In fact, OFDM has been chosen for European digital audio and video broadcasting, and wireless local area network (WLAN) standards, such as, for example, 802.11a.
In single user OFDM systems, techniques such as the water-filling approach may be utilized to determine an appropriate sub-carrier and bit allocation solution that minimizes total transmit power. In multi-user OFDM systems, however, such determinations are much more difficult. Sub-carriers in a multi-user system, for example, may be desirable to more than one user at the same time. Such variables add a great deal of complexity to the determining of optimal power allocation schemes, i.e., the scheduling of non-real-time (NRT) services.
Existing approaches directed at scheduling NRT services in multi-user systems utilize a non-linear optimization approach, such as, for example, the Lagrangian heuristic procedure with relaxation. These non-linear approaches, however, require intensive computations and are incapable of yielding optimal solutions. At most, such approaches yield only lower and upper bounds.
None of the existing approaches, including the non-linear approaches discussed above, consider ‘fairness’ between users in scheduling NRT services. Fairness, as further described below, is an indication of the quality of services (QoS) experienced by one user in the system versus the QoS expected, or the QoS experienced by other users in the system. By not considering fairness, the QoS experienced by certain users will be far superior to the QoS experienced by other users.
To illustrate, consider FIG. 1. In a multi-user OFDM network 100, base station 110 is servicing wireless transmit/receive units (WTRUs) 102, 104, and 106. Each WTRU has a respective channel gain Gk,n on a particular sub-carrier, where k represents the sub-carrier and n represents the user, or in this case, the WTRU. As illustrated in the Figure, WTRU 102 has a channel gain on sub-carrier k equivalent to Gk,102. Similarly, WTRUs 104 and 106 have channel gain values of Gk,104 and Gk,106, respectively. If, for example, WTRU 102 has a higher channel gain value Gk,102 than the other WTRUs 104, 106, sub-carrier k will be allocated to WTRU 102. As long as WTRU 102 continues to have a channel gain value Gk,102 superior to that of WTRUs 104 and 106, it will continue to occupy sub-carrier k, and thus continue to experience superior system performance. In the mean time, WTRUs 104 and 106 continue to suffer sub-par performance at the expense of WTRU 102. If fairness between users were utilized in system 100, however, WTRUs 102, 104 and 106 would all experience comparable system performance, in spite of their individual channel gain values.
Accordingly, it is desirable to have a method and apparatus for scheduling NRT services in a multi-user wireless OFDM system that considers and maintains fairness between users and does not require intense computations.